This invention relates to synthetic lubricant compositions. In zeolite catalyzed oligomerization of propylene or other lower olefins to produce high viscosity index (HVI) lubricant range hydrocarbons in the C.sub.20 -C.sub.60 range by shape selective catalysis, it has been observed that the average molecular weights of the lube products that give viscosities greater than 6 cS at 100.degree. C. are not easily obtainable, due to diffusion limitation imposed by the medium pore catalyst structure. While these low cost lubricants can be made by the Mobil Olefins to Lubricants ("MOL") process, it may be necessary to add viscosity improvers to obtain acceptable lubricant formulations. Synthetic hydrocarbon fluids have found increasing use as lubricant basestocks, additives and functional fluids. Automotive lubricants based on .alpha.-olefin oligomers have been commercially available for over a decade, preceded by many years of research to develop economic synthetic oils with improved viscosity index (VI), volatility, oxidation stability and lower temperature fluidity than naturally occurring mineral oils or those produced from refining of petroleum. Particular attention has been directed to upgrading low cost refinery olefins, such as C.sub.3 -C.sub.4 byproducts of heavy oil cracking processes. Work by Garwood, Chen, Tabak and others has led to development of a useful process for producing lubricant range hydrocarbons by shape selective catalysis using medium pore ZSM-5 by the "MOL" process described herein.
Synthetic poly-alpha-(.alpha.-)olefins (PAO), such as 1-decene oligomers, have found wide acceptability and commercial success in the lubricant field for their superiority to mineral oil based lubricants. In terms of lubricant properties improvement, industrial research effort on synthetic lubricants has led to PAO fluids exhibiting useful viscosities over a wide range of temperature, i.e., improved viscosity index (VI), while also showing lubricity, thermal and oxidative stability and pore point equal to or better than mineral oil. These relatively new synthetic lubricants lower mechanical friction, enhancing mechanical efficiency over the full spectrum of mechanical loads from worm gears to friction drives and do so over a wider range of ambient operating conditions than mineral oil. The PAO's are prepared by the polymerization of 1-alkenes using typically Lewis acid or Ziegler-catalysts. Their preparation and properties are described by J. Brennan in Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, pp 2-6, incorporated herein by reference in its entirety. PAO incorporating improved lubricant properties are also described by J. A. Brennan in U.S. Pat. Nos. 3,382,291, 3,742,082, and 3,769,363, incorporated herein by reference.
In accordance with customary practice in the lubricants art, PAO's have been blended with a variety of functional chemicals, oligomeric and high polymers and other synthetic and mineral oil based lubricants to confer or improve upon lubricant properties necessary for applications such as engine lubricants, hydraulic fluids, gear lubricants, etc. Blends and their components are described in Kirk-Othmer Encyclopedia of Chemical Technology, third edition, volume 14, pages 477-526, incorporated herein by reference. A particular goal in the formulation of blends is the enhancement of viscosity index (VI) by the addition of VI improvers which are typically high molecular weight synthetic organic molecules. While effective in improving viscosity index, these VI improvers have been found to be deficient in that their very property of high molecular weight that makes them useful as VI improvers also confers vulnerability in shear stability to the blended materials during actual use applications. This deficiency dramatically negates the range of application usefulness for many VI improvers. Their usefulness is further compromised by cost since they are relatively expensive polymeric substances that may constitute a significant proportion of the final lubricant blend. Accordingly, workers in the lubricant arts continue to search for lubricant blends with high viscosity index less vulnerable to degradation by shearing forces in actual use applications while maintaining other important properties such as thermal and oxidative stability.
Blending the conventional low viscosity PAO with MOL type oligomers, as described above, produces mixtures which have aggregative properties of the blended components.
Recently, a novel class of PAO lubricant liquid compositions, herein referred to as "HVI-PAO", exhibiting surprisingly high viscosity indices has been reported by M. M. Wu in U.S. Pat. Nos.4,827,064 and 4,827,073, incorporated herein by reference. These novel PAO lubricants are particularly characterized by low ratio of methyl to methylene groups, i.e., low branch ratios, as further described hereinafter. Their very unique structure provides new opportunities for the formulation of distinctly superior and novel lubricant blends. It has been found that these HVI-PAO type synthetic polymeric components, when admixed with relatively low viscosity MOL type oligomeric base stock oil, provides greatly enhanced VI of the blend of materials along with shear stability. This enhanced viscosity property is substantially greater than would be expected from a knowledge of the properties of the individual components. Accordingly, it is an object of the present invention to provide novel lubricant compositions having improved viscosity index and shear stability. It is a further object of the present invention to provide novel lubricant basestock blends from low viscosity synthetic MOL liquids and high viscosity PAO and HVI-PAO. In conjunction with a major amount of the MOL liquid hydrocarbons, the PAO additives provide excellent chemical and physical properties.